1. Field of the Invention
The present invention relates to an optimum control method and apparatus for internal combustion engines, which feedback-control the ignition timing, air-fuel ratio and fuel supply quantity of an engine so as to improve the output and fuel consumption rate of the engine.
2. Description of the Prior Art
The ignition timing for most internal combustion engines is adjusted, unless there exists any special reason such as the requirements from knocking or exhaust gas characteristic, in accordance with the operating conditions of the engine, namely, on the basis of an engine speed, intake pressure, etc., so as to simultaneously attain a maximum output and a minimum fuel consumption rate for the engine. However, the effect of these prior art methods is limited and they inevitably suffer some losses in both engine output and fuel consumption rate. For instance, such losses are caused by such factors as the variations in performance among manufactured engines, the variations in ignition timing correction, the changes in ambient conditions, etc.
Techniques have been devised for feedback-controlling the ignition timing of an engine so as to eliminate such losses and provide a maximum engine output. In an example of these ignition timing feedback control techniques, the ignition timing of an engine is controlled at an optimum ignition timing which provides a maximum engine torque by selecting at least two ignition timings which are different from each other and are in the vicinity of but apart by a given ignition angle from a calculated ignition timing obtained in accordance with the operating conditions of the engine, operating the engine at the at least two selected ignition timings alternately for a given time period, detecting a signal indicative of the engine speed when the engine is operated at each of the at least two ignition timings, comparing the engine speed signals with one another which have been obtained at at least three successive operating points during the operation of the engine at each of the at least two selected ignition timings, deciding whether the calculated ignition timing is advanced or retarded from an optimum ignition timing (the minimum spark advance for best torque or MBT) and then correcting the calculated ignition timing in accordance with the result of the decision.
In accordance with this technique in which a change in the output is determined on the basis of a change in the engine speed, although the engine speed varies naturally due to various factors, it is possible to discriminate whether the actual change in the engine speed is due to the variation of the ignition timing or any other external factor (e.g., an accelerator pedal operation) and to prevent erroneous correction control of the ignition timing in a direction opposite to the optimum ignition timing by interrupting the ignition timing correction during acceleration and deceleration periods, slope ascending and desceding periods, etc.
With the above-described prior art control technique, the direction toward an optimum ignition timing can be decided without any difficulty in cases where the difference between a calculated ignition timing and the optimum ignition timing is sufficiently large. However, the smaller the difference becomes, the more difficult it becomes to decide whether an engine speed change is due to a change of the ignition timing or any other factors such as variations in the air-fuel ratio and the flame propagation speed in respective combustions occurring in the engine, etc.
As a result, when a calculated ignition timing comes near the optimum ignition timing, the ignition timing may be corrected in response to the detection of a change of the engine speed due to such any other factors in spite that there is no necessity of making any further correction.
In the description to follow, the operation of intentionally varying a control variable for controlling the operation of the engine and deciding the resulting change in the operating condition thereby to effect the optimum control of the engine is referred to as "dither" and the period of time during which the dither is effected is referred to as a "dither period". Also, an amount of change of the control variable for effecting the dither is referred to as a "dither quantity".
Further, an internal combustion engine is hereinafter simply referred to as an engine.
The present invention has been made with a view to solving the foregoing problems in the prior art.